This invention relates to a distillation process for purifying crude C.sub.1 -C.sub.3 alkanols. More particularly, this invention relates to an improved process for preparing pure C.sub.1 -C.sub.3 alkanols using aqueous alkali at a certain stage of a three zone distillation operation.
Saturated C.sub.1 -C.sub.3 crude alcohols obtained by various methods through dry distillation, fermentation or synthesis, are contaminated with substances such as ethers, acids, aldehydes, ketones and esters, the removal of which often entails considerable difficulty.
Conventional purification processes previously used were generally such that the alcohol-containing fluid to be purified was introduced into a distillation column into whose bottom end was fed the amount of steam required for the removal of the impurities. The impurities were then removed at the top of the zone, while the partially purified alcohol was drawn out at a lower level and subjected to further purification in a rectifying apparatus. The impurities fraction however still contain appreciable amounts of alcohol which is usually difficult to separate.
In the prufication of alcohol originating from the catalytic hydration of ethylene, it is also possible to proceed by first adjusting the pH value of the reaction product to between 6 and 9 by the addition of alkaline substances, then performing a fractional distillation, hydrogenating the ethanol solution thus obtained in the presence of a nickel catalyst, and then again distilling it fractionally. While this process is complicated and expensive, the first process mentioned has the main disadvantage that considerable amounts of low-grade alcohols are produced. It has therefore been suggested that the purification be performed in the following manner.
The crude alcohol is fed into the middle of a zone, the amount of steam required for the extraction of the impurities is injected into the lower part, and pure water or water containing a small amount of alcohol is sprinkled in the upper part of the zone, in such a manner that the alcohol concentration in the upper part of the zone does not exceed 20% by volume. Then the impurities are drawn off at the top of the zone in concentrated form and returned to the top tray of the zone, although a portion of the condensate is drawn off in such a manner that the amount removed corresponds to the amount of impurities contained in the starting liquid. At the upper part of the zone, then, a zone of highly concentrated impurities forms.
This process can be improved by providing a decanting tray at the top of the column in order to achieve a separation of the liquid that gathers there into an upper layer rich in impurities, which is drawn off, and a lower layer having a lower content of impurities. In this manner it is said to be possible to achieve yields of more than 99.5% of pure alcohol.
Actually, however, it has been found that the process is not suitable for the purification of highly contaminated synthesis alcohols containing particularly great amounts of ether and aldehydes, because separation of the liquids by decantation is not possible since they do not separate sufficiently by gravity.
For the prevention of corrosion phenomena, and to improve the purifying action, it is furthermore desirable to add alkali to the sprinkling water which comes from the bottom part of a second zone and is recirculated to the head of the first zone.
In this case, however, resin-like products are formed due to the aldehyde content of the crude alcohol, some of them being deposited in the form of incrustations on the trays of the zone and resulting in an impairment of the action of the zone.
These phenomena are particularly pronounced when the starting ethylene has not been subjected to any separate hydrogenation prior to the synthesis, because the acetylene contained in the ethylene is largely transformed to acetaldehyde, and then the above-described severe incrustation and resinification develops, which furthermore renders safe and reliable operation impossible. It is therefore necessary to shut down the plant from time to time and perform a difficult and expensive cleaning operation.
A process is also known for obtaining alcohol from mixtures containing crude alcohol, which is characterized by the fact that into the middle section of a first distillation tower, which forms a zone for extraction distillation, there is introduced an input current containing a saturated, aliphatic alcohol of no more than three carbon atoms containing lower and higher boiling impurities as accompanying substances, water is introduced into the upper section of the first distillation tower, a current containing virtually all the impurities is drawn from the head of the first distillation tower, a dilute aqueous current containing the alcohol in concentrations of about 5 weight percent to about 10 weight percent plus smaller amounts of the lower and higher boiling accompanying substances is taken from the bottom of the first distillation tower, the dilute, aqueous current taken from the bottom of the first distillation tower is delivered to a second distillation tower forming a rectification or alcohol concentration zone, an alcohol product current containing the alcohol in high concentration and in purified form is taken from the upper section of the second distillation tower, a purification current containing the lower boiling impurities is taken from the head of the second distillation tower, at least one purification current containing the higher boiling impurities is taken from the middle section of the second distillation tower, the purification currents from the second distillation tower and the current taken from the head of the first distillation tower are transferred to a third distillation tower forming a zone for the concentration of the impurities, the lower and higher boiling impurities are taken from the third distillation tower, and by the fact that a current containing alcohol is taken from the third distillation tower and recycled together with the input current to the first distillation tower. The products of this process, however, fail to meet the purity requirements of a number of applications. Such requirements can, however, be met by proceeding in accordance with the present invention.